This invention relates to cutting elements and more specifically to cutters having a non-planar interface between their substrate and cutting layer, e.g. cutting table.
For descriptive purposes the present invention is described in terms of a cutter. A cutter, shown in FIG. 30 typically has a cylindrical cemented carbide substrate body 100 having a longitudinal axis 102. A diamond cutting table (i.e., diamond layer) 34 is bonded onto the substrate. The cutting table has a planar, typically horizontal upper surface 103. As it would become apparent to one skilled in the art, the invention described herein could easily be applied to other types of cutting elements such as enhanced cutters, end mills, drills and the like. Moreover, "diamond," "diamond surface" and "diamond table" are used interchangeably herein to describe the cutter cutting table.
Common problems that plague cutting elements and specifically cutters having an ultra hard diamond-like cutting table such as polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) bonded on a cemented carbide substrate are chipping, spalling, partial fracturing, cracking or exfoliation of the cutting table. These problems result in the early failure of the cutting table and thus, in a shorter operating life for the cutter.
It has been thought that the problems, i.e., chipping, spalling, partial fracturing, cracking, and exfoliation of the diamond layer are caused by the difference in the coefficient of thermal expansion between the diamond and the substrate. Specifically, the problems are thought to be caused by the abrupt shift in the coefficient of thermal expansion on the interface 104 between the substrate and the diamond. This abrupt shift causes the build-up of residual stresses on the cutting layer.
The cemented carbide substrate has a higher coefficient of thermal expansion than the diamond. During sintering, both the cemented carbide body and diamond layer are heated to elevated temperatures forming a bond between the diamond layer and the cemented carbide substrate. As the diamond layer and substrate cool down, the substrate shrinks more than the diamond because of its higher coefficient of thermal expansion. Consequently, stresses referred to as thermally induced stresses are formed at the interface between the diamond and the body.
Moreover, residual stresses are formed on the diamond layer from decompression after sintering. The high pressure applied during the sintering process causes the carbide to compress more than the diamond layer. After the diamond is sintered onto the carbide and the pressure is removed, the carbide tries to expand more than the diamond imposing a tensile residual stress on the diamond layer.
In an attempt to overcome these problems, many have turned to use of non-planar interfaces between the substrate and the cutting layer. The belief being, that a non-planar interface allows for a more gradual shift in the coefficient of thermal expansion from the substrate to the diamond table, thus, reducing the magnitude of the residual stresses on the diamond. Similarly, it is believed that the non-planar interface allow for a more gradual shift in the compression from the diamond layer to the carbide substrate. However, these non-planar interfaces do not address all of the problems that plague cutters.
Another reason for cracking and also for the spalling, chipping and partial fracturing of the diamond cutting layer is the generation of peak (high magnitude) stresses generated on the diamond layer on the region at which the cutting layer makes contact with the earthen formation during cutting. Typically, the cutters are inserted into a drag bit at a rake angle. Consequently, the region of the cutter that makes contact with the earthen formation includes a portion of the diamond layer near to and including the diamond layer circumferential edge.
A yet further problem with current cutters is the delamination and/or exfoliation of the diamond layer from the substrate of the cutter resulting in the failure of the cutter. Delamination and/or exfoliation become more prominent as the thickness of the diamond layer increases.
Another disadvantage with some current cutters having non-planar interfaces, is that they must be installed in the drag bits in a certain orientation. For example, cutters which have a non-planar interface consisting of alternating ridges and grooves, must be positioned on the drag bit such that the alternating ridges and grooves are perpendicular to the earth formation 14 (FIG. 31). The rationale being that as the cutter wears, the diamond located in the grooves on the substrate will be available to assist in cutting. Consequently, the installation of such cutters on a drag bit at a specific orientation becomes time consuming thereby, increasing the cost of drilling operations.
Accordingly, there is a need for a cutter having a diamond table with improved cracking, chipping, fracturing, and exfoliating characteristics, and thereby an enhanced operating life which is not orientation dependent when inserted into a drag bit.